Electrodiagnostic Study Instrument Design Requirements

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Introduction  

Electrodiagnostic (EDX) physicians rely upon quality nerve conduction studies (NCSs) and needle electromyography (EMG) instruments to diagnose neuromuscular disorders. Instrumentation has changed due to improved electronic technology and new research findings in EDX medicine. This statement is intended to define the requirements and specifications of a useful EDX instrument.

Electrical Safety 

Safety standards are required for any electrical medical device. The EDX instruments should follow FDA standards for safety, including acceptable current leakage requirements.1 The EDX equipment should be routinely maintained per the manufacturer’s guidelines.

Data Integrity 

An EDX instrument should provide password-controlled access to maintain patient record confidentiality. The consistent use of unique patient identifiers should be employed across systems including the electrodiagnostic instrument.

EDX Instrument Design 
 
The purpose of an EDX instrument is to objectively record, amplify, display, and store low-amplitude neurophysiological signals despite the expected presence of ambient noise, interference and stimulus artifacts. EDX instruments have three separate functional components:  signal input, processing, and output.    
 

I.Signal Input:  Electrodes
 

Stimulating Electrode (Stimulator)

  • cathode and anode clearly identified
  • support for monopolar needle stimulation
  • continuously adjustable intensity
  • adjustable stimulus duration from 0.05 to 1 millisecond (ms)
  • adjustable stimulation frequency from 0.1 to 50 Hertz (Hz) 2-4
     
 
 Recording Electrodes
  • connections for three electrodes: active, reference, and ground
  • connections being touch-proof for patient safety
  • electrode design allowing for anatomical variation
  • ability to use surface and needle recording electrodes for NCSs
  • ability to use monopolar and concentric needle electrodes for needle EMG

 
 

Temperature indicator and temperature probe

  • built in or an external temperature probe
 
  
II.Signal Processing
 
 Differential amplifier

A differential amplifier magnifies the potential difference between the active and reference inputs to improve signal-to-noise ratio. 
  • high input impedance of the amplifier (>1,000M-Ohms) 
  • high common-mode rejection ratio (CMRR), e.g. >100 dB 
  • noise level with input shorted being less than 0.6 µVRMS
  • channel selection mechanism if multiple channels are needed
 
 Gain (Sensitivity)
  • ability to acquire signals from 1 microvolt (µV) to 50 millivolts (mV) 2-4 
  • analog gain stages with minimum of 3 analog gains (digital amplification increases noise significantly and can mask the biological signal.) 
 
 Filters
All amplifiers use a band-pass filter to attenuate noise. The band-pass filter is characterized by adjustable low and high cut-off frequency settings.
  • adjustable lower frequency (high pass filter) setting from 1 to 2,000 Hz 
  • adjustable high frequency (low pass filter) setting from 100 to 10,000 Hz 2-5 
  • notch filter for 50 Hz or 60 Hz for noise elimination (This is to attenuate the power line frequency but should not be active by default because of potential amplitude reduction and ringing.)
 
 Analog-to-digital converter
  • converts biological (analog) signals to digital waveforms
  • ability of displaying and storing waveforms in a digital format
  • adequate sampling frequency to prevent waveform distortion from aliasing
 
III.Signal Output
 
 Signal display
  • sensitivity/gain control to determine the amplitude of potentials, with a range of 1 µV to 10 mV per division 
  • sweep speed adjustment, with a range of 0.1 to 500 ms per division 
  • trace area appearance of a rectangular grid, with the gain and sweep speed clearly labeled 
  • adequate vertical and horizontal resolution on monitor to enable visual analysis of waveforms   
  • automatic cursor placement at onset and peak of recognizable potentials, to measure latencies (onset and peak) and amplitudes (baseline-to-peak and peak-to-peak) 
  • manual adjustment of cursors for measurements during both NCSs and EMG testing 
  • automatic calculation of conduction velocity when distance values are entered    
  • free running and triggered mode
 The free running mode updates signal display continuously, showing live electrophysiological signals as they are recorded. The triggered mode is necessary to record signals when a certain event (the trigger) occurs to assess signal variability and reproducibility. For triggered modes, the occurrence of the event (stimulus) should be synchronized to the acquisition of data point for accurate time zero calculation. For motor unit potential analysis, an adjustable level trigger should be available. A function of window triggering is optional. A function of delay line with adjustable delay time should be available to allow observation and analysis of signals preceding the trigger. 

 
 
  • capability of trace raster/superimpose
  • square wave calibration signal to calibrate gain, sweep and other functions  
 Auditory Speaker
  • high quality audio amplifier and loudspeakers for the production of characteristic sounds, for both potential recognition and criterion analysis 
  • volume adjustment
 Data storage and report generation 
  • Exports numerical data directly into the final report 
 Data are objective and based on real time measurement of biological signals without subjective input from the patient and are independent of psychophysical responses from patients. 
 
  • Report editing
  • Lists normal reference values
  • Allows for inclusion of acquired waveforms
 
Conclusion

The EDX instrument should provide the original numerical NCS/EMG data delineated in the AANEM position statement “Reporting the Results of Needle EMG and Nerve Conduction Studies: An Educational Report.” 7 The updated version includes an option to specify the EDX instrument manufacturer and model on the report.8
Finally, efficient usage of EDX instruments requires performance by or oversight of an appropriately trained EDX physician. The AANEM position statement “Who is Qualified to Practice Electrodiagnostic Medicine” defines recommended qualifications for an EDX physician.9 

 

References

  1. FDA-recognized standard, IEC 60601-1, “Medical Electrical Equipment - Part 1: General Requirements for Safety.”  
  2. Oh SJ., Clinical electromyography, nerve conduction studies, Chapter 3, Basic components of electromyography instruments, 3rd ed, Philadelphia, Lippicott Williams & Wilkins; 2003, p. 25-36.  
  3. Oh SJ. Principles of Clinical Electromyography, Case Studies, Chapter 2, Basic components of the EMG machine and its setups for testing, Philadelphia, Lippicott Williams & Wilkins; 1998, p.13-20.  
  4. Dumitru D, Zwarts MJ. Instrumentation. In: Dumitru D, Amato A, Zwarts MJ, editors. Electrodiagnostic medicine, 2nd ed. Philadelphia, Hanley & Belfus, 2002. p. 69-97.  
  5. Nandedkar SD. Chapter 5, Instrumentation. In: Pease WS, Lew HL, Johnson EW, editors. Johnson's Practical Electromyography.  Philadelphia, Lippincott Williams & Wilkins, 2007, p. 87-103.
  6. Preston DC, Shapiro BE, Chapter 3, Basic nerve conduction studies. In: Preston DC, Shapiro BE, editors Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic Correlations, 3rd edition. New York, Elsevier Sanders, 2013, p. 19-35.
  7. AANEM Position Statement. Reporting the Results of Nerve Conduction Studies and Needle EMG.
  8. AANEM Position Statement.  Model Report.
  9. AANEM Position Statement. Who is Qualified to Practice Electrodiagnostic Medicine.

 (Links to AANEM position statements above are to the most recent version of the document.)

Document History

Approved by the American Association of Neuromuscular & Electrodiagnostic Medicine Board: July 2015 and December 2020.

 
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